In many electronic systems, it is desirable to measure electric currents flowing through a load. One such application is consumer audio devices, where it is desirable to measure the load current flowing to a speaker from a power amplifier. The measured current information is useful for determining the load on the amplifier for the purposes of determining overload conditions, or abnormal conditions such as short circuit loads that can damage the power amplifier.
Typically, the current measurement is made by placing a resistor in series with the load, and measuring the voltage drop across this resistor. The voltage drop can be converted to a measured current using Ohm's law. The voltage drop across the measurement resistor is minimized to reduce the impact of the measurement on the final voltage delivered to the load. A differential operational amplifier is often used to measure the very small voltage drop. The differential amplifier must have very good “common mode rejection” specifications, however, to accurately make this measurement.
Common mode rejection is the ability of an amplifier to subtract the effect of the same voltage applied to both its non-inverting and inverting inputs. An ideal amplifier would have an infinite common mode rejection ratio, resulting in no effect at its output if the same voltage were applied to its two inputs. In actual practice, a number of factors limit the use of such amplifiers. Firstly, these amplifiers can be expensive. Secondly, common mode rejection ratings of operational amplifiers are frequency dependent, often decreasing as frequency increases. This makes them less desirable for AC and complex audio signal measurements. Thirdly, the range of signal levels is limited to the power supply voltages of the measuring amplifier, making measurements of higher voltage signals found in power audio applications difficult or impractical.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.